this is the main heading
this text might be an introduction to the rest of the page ,and if the page is a long one
this is a sub-heading
Many long acticles have sub_heading
Another Sub-Heading
Here you can see another sub-Heading
网页的组成
文本
图片 jpg， png
音频 mp3
视频 mp4
其中，HTML源码编辑器本身还有文件类型，文件类型的语言种类等元素，故可以省去<head></head>之间的代码